Some unsteady features of turbulent boundary layers

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Abstract

For steady free-stream, zero and favorable pressure gradient turbulent
boundary layers, the unsteadiness in the form of turbulent fluetuations was
investigated. Phase ensemble-averaged flow characteristics of a large amplitude
periodic unsteady turbulent boundary layer was also investigated at a redueed
frequency k = 0.61 based on the length of the eonverging and diverging test
section with amplitude to mean velocity ratio of 0.8.
ln steady flow cases, both zero and favorable pressure gradient flows show
good two—dimensional flow characteristics and mean flow characteristics are
compared with other researchers’ data. Measured power spectral data show good
agreement with those of Klebanoff, Ueda and Hinze, Perry, Lim and Henbest for
the zero pressure gradient flows and Jones and Launder for the favorable
pressure gradient flow. The power spectral data measured in the turbulent wall
region of the zero pressure gradient flow closely follow the model equation
proposed by Perry, Lim and Henbest. Convective wave speed also show good
agreement with those of Favre, Gaviglio and Dumas and Sternberg within the
experimental uncertainties. ln the inner region of the boundary layer where
y+ < 40, convective wave speed is higher than local mean velocity at all eddy
scales as observed by Kline, Reynolds, Schraub and Runstadler.
In the unsteady flow case, in the absence of flow reversal, the flow behaves
in a quasi-steady manner and can be described by the steady flow structure as in
the case of moderate amplitude flows. The Ludwieg·Tillmann skin friction
equation and the Perry-Schofield universal velocity defect law hold at these
phases. Except the laminariscent velocity profile observed during the acceleration
phases, the large amplitude unsteady flow shows basically the same flow
characteristics as the moderate amplitude flows.